[PATCH net-next v2] net: sctp: keep owned chunk in destructor_arg instead of skb->cb

[PATCH net-next v2] net: sctp: keep owned chunk in destructor_arg instead of skb->cb

[Daniel Borkmann]

It's just silly to hold the skb destructor argument around inside
skb->cb[] as we currently do in SCTP.

Nowadays, we're sort of cheating on data accounting in the sense
that due to commit 4c3a5bdae293 ("sctp: Don't charge for data in
sndbuf again when transmitting packet"), we orphan the skb already
in the SCTP output path, i.e. giving back charged data memory, and
use a different destructor only to make sure the sk doesn't vanish
on skb destruction time. Thus, cb[] is still valid here as we
operate within the SCTP layer. (It's generally actually a big
candidate for future rework, imho.)

However, storing the destructor in the cb[] can easily cause issues
should an non sctp_packet_set_owner_w()'ed skb ever escape the SCTP
layer, since cb[] may get overwritten by lower layers and thus can
corrupt the chunk pointer. There are no such issues at present,
but lets keep the chunk in destructor_arg, as this is the actual
purpose for it.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
---
 v1->v2:
  - Only reworded commit message to make it more clear

 net/sctp/socket.c | 12 ++++--------
 1 file changed, 4 insertions(+), 8 deletions(-)

diff --git a/net/sctp/socket.c b/net/sctp/socket.c
index 2120292..85e0b65 100644
--- a/net/sctp/socket.c
+++ b/net/sctp/socket.c
@@ -162,7 +162,7 @@ static inline void sctp_set_owner_w(struct sctp_chunk *chunk)
 
 	chunk->skb->destructor = sctp_wfree;
 	/* Save the chunk pointer in skb for sctp_wfree to use later.  */
-	*((struct sctp_chunk **)(chunk->skb->cb)) = chunk;
+	skb_shinfo(chunk->skb)->destructor_arg = chunk;
 
 	asoc->sndbuf_used += SCTP_DATA_SNDSIZE(chunk) +
 				sizeof(struct sk_buff) +
@@ -6870,14 +6870,10 @@ static void sctp_wake_up_waiters(struct sock *sk,
  */
 static void sctp_wfree(struct sk_buff *skb)
 {
-	struct sctp_association *asoc;
-	struct sctp_chunk *chunk;
-	struct sock *sk;
+	struct sctp_chunk *chunk = skb_shinfo(skb)->destructor_arg;
+	struct sctp_association *asoc = chunk->asoc;
+	struct sock *sk = asoc->base.sk;
 
-	/* Get the saved chunk pointer.  */
-	chunk = *((struct sctp_chunk **)(skb->cb));
-	asoc = chunk->asoc;
-	sk = asoc->base.sk;
 	asoc->sndbuf_used -= SCTP_DATA_SNDSIZE(chunk) +
 				sizeof(struct sk_buff) +
 				sizeof(struct sctp_chunk);
-- 
1.7.11.7

Re: [PATCH net-next v2] net: sctp: keep owned chunk in destructor_arg instead of skb->cb

[Vlad Yasevich]

On 11/19/2014 07:54 PM, Daniel Borkmann wrote:
> It's just silly to hold the skb destructor argument around inside
> skb->cb[] as we currently do in SCTP.
> 
> Nowadays, we're sort of cheating on data accounting in the sense
> that due to commit 4c3a5bdae293 ("sctp: Don't charge for data in
> sndbuf again when transmitting packet"), we orphan the skb already
> in the SCTP output path, i.e. giving back charged data memory, and
> use a different destructor only to make sure the sk doesn't vanish
> on skb destruction time. Thus, cb[] is still valid here as we
> operate within the SCTP layer. (It's generally actually a big
> candidate for future rework, imho.)
> 
> However, storing the destructor in the cb[] can easily cause issues
> should an non sctp_packet_set_owner_w()'ed skb ever escape the SCTP
> layer, since cb[] may get overwritten by lower layers and thus can
> corrupt the chunk pointer. There are no such issues at present,
> but lets keep the chunk in destructor_arg, as this is the actual
> purpose for it.
> 
> Signed-off-by: Daniel Borkmann <dborkman@redhat.com>

Good clean-up.

Acked-by: Vlad Yasevich <vyasevich@gmail.com>

-vlad

> ---
>  v1->v2:
>   - Only reworded commit message to make it more clear
> 
>  net/sctp/socket.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index 2120292..85e0b65 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -162,7 +162,7 @@ static inline void sctp_set_owner_w(struct sctp_chunk *chunk)
>  
>  	chunk->skb->destructor = sctp_wfree;
>  	/* Save the chunk pointer in skb for sctp_wfree to use later.  */
> -	*((struct sctp_chunk **)(chunk->skb->cb)) = chunk;
> +	skb_shinfo(chunk->skb)->destructor_arg = chunk;
>  
>  	asoc->sndbuf_used += SCTP_DATA_SNDSIZE(chunk) +
>  				sizeof(struct sk_buff) +
> @@ -6870,14 +6870,10 @@ static void sctp_wake_up_waiters(struct sock *sk,
>   */
>  static void sctp_wfree(struct sk_buff *skb)
>  {
> -	struct sctp_association *asoc;
> -	struct sctp_chunk *chunk;
> -	struct sock *sk;
> +	struct sctp_chunk *chunk = skb_shinfo(skb)->destructor_arg;
> +	struct sctp_association *asoc = chunk->asoc;
> +	struct sock *sk = asoc->base.sk;
>  
> -	/* Get the saved chunk pointer.  */
> -	chunk = *((struct sctp_chunk **)(skb->cb));
> -	asoc = chunk->asoc;
> -	sk = asoc->base.sk;
>  	asoc->sndbuf_used -= SCTP_DATA_SNDSIZE(chunk) +
>  				sizeof(struct sk_buff) +
>  				sizeof(struct sctp_chunk);
> 

Re: [PATCH net-next v2] net: sctp: keep owned chunk in destructor_arg instead of skb->cb

[Neil Horman]

On Thu, Nov 20, 2014 at 01:54:48AM +0100, Daniel Borkmann wrote:
> It's just silly to hold the skb destructor argument around inside
> skb->cb[] as we currently do in SCTP.
> 
> Nowadays, we're sort of cheating on data accounting in the sense
> that due to commit 4c3a5bdae293 ("sctp: Don't charge for data in
> sndbuf again when transmitting packet"), we orphan the skb already
> in the SCTP output path, i.e. giving back charged data memory, and
> use a different destructor only to make sure the sk doesn't vanish
> on skb destruction time. Thus, cb[] is still valid here as we
> operate within the SCTP layer. (It's generally actually a big
> candidate for future rework, imho.)
> 
> However, storing the destructor in the cb[] can easily cause issues
> should an non sctp_packet_set_owner_w()'ed skb ever escape the SCTP
> layer, since cb[] may get overwritten by lower layers and thus can
> corrupt the chunk pointer. There are no such issues at present,
> but lets keep the chunk in destructor_arg, as this is the actual
> purpose for it.
> 
> Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
> ---
>  v1->v2:
>   - Only reworded commit message to make it more clear
> 
>  net/sctp/socket.c | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
> 
> diff --git a/net/sctp/socket.c b/net/sctp/socket.c
> index 2120292..85e0b65 100644
> --- a/net/sctp/socket.c
> +++ b/net/sctp/socket.c
> @@ -162,7 +162,7 @@ static inline void sctp_set_owner_w(struct sctp_chunk *chunk)
>  
>  	chunk->skb->destructor = sctp_wfree;
>  	/* Save the chunk pointer in skb for sctp_wfree to use later.  */
> -	*((struct sctp_chunk **)(chunk->skb->cb)) = chunk;
> +	skb_shinfo(chunk->skb)->destructor_arg = chunk;
>  
>  	asoc->sndbuf_used += SCTP_DATA_SNDSIZE(chunk) +
>  				sizeof(struct sk_buff) +
> @@ -6870,14 +6870,10 @@ static void sctp_wake_up_waiters(struct sock *sk,
>   */
>  static void sctp_wfree(struct sk_buff *skb)
>  {
> -	struct sctp_association *asoc;
> -	struct sctp_chunk *chunk;
> -	struct sock *sk;
> +	struct sctp_chunk *chunk = skb_shinfo(skb)->destructor_arg;
> +	struct sctp_association *asoc = chunk->asoc;
> +	struct sock *sk = asoc->base.sk;
>  
> -	/* Get the saved chunk pointer.  */
> -	chunk = *((struct sctp_chunk **)(skb->cb));
> -	asoc = chunk->asoc;
> -	sk = asoc->base.sk;
>  	asoc->sndbuf_used -= SCTP_DATA_SNDSIZE(chunk) +
>  				sizeof(struct sk_buff) +
>  				sizeof(struct sctp_chunk);
> -- 
> 1.7.11.7
> 
> --
> To unsubscribe from this list: send the line "unsubscribe linux-sctp" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 
Acked-by: Neil Horman <nhorman@tuxdriver.com>

Re: [PATCH net-next v2] net: sctp: keep owned chunk in destructor_arg instead of skb->cb

[David Miller]

From: Daniel Borkmann <dborkman@redhat.com>
Date: Thu, 20 Nov 2014 01:54:48 +0100

> It's just silly to hold the skb destructor argument around inside
> skb->cb[] as we currently do in SCTP.
> 
> Nowadays, we're sort of cheating on data accounting in the sense
> that due to commit 4c3a5bdae293 ("sctp: Don't charge for data in
> sndbuf again when transmitting packet"), we orphan the skb already
> in the SCTP output path, i.e. giving back charged data memory, and
> use a different destructor only to make sure the sk doesn't vanish
> on skb destruction time. Thus, cb[] is still valid here as we
> operate within the SCTP layer. (It's generally actually a big
> candidate for future rework, imho.)
> 
> However, storing the destructor in the cb[] can easily cause issues
> should an non sctp_packet_set_owner_w()'ed skb ever escape the SCTP
> layer, since cb[] may get overwritten by lower layers and thus can
> corrupt the chunk pointer. There are no such issues at present,
> but lets keep the chunk in destructor_arg, as this is the actual
> purpose for it.
> 
> Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
> ---
>  v1->v2:
>   - Only reworded commit message to make it more clear

Applied, thanks.